The present invention relates to the fabrication of turbine blade assemblies and, in particular, to an improved shim for controlling turbine blade motion.
A typical turbine blade row is composed of a plurality of individual blades which are installed in a support member, typically an annular rotor or hub. The conventional turbine blade includes a root portion by which the blade is connected to the hub in a free-standing manner whereby each blade is supported only at this connection. When blades are assembled to the hub, movement of each blade relative to the hub in the tangential direction of the rotor must be eliminated to the greatest extent possible. Typically, when a turbine is rotating at high speed, centrifugal forces act to stabilize the position of each blade relative to the hub. However, at lower speeds, such as turning gear speeds, there is a tendency for the blade roots to move circumferentially within their respective rotor grooves. Specifically, as the turbine assembly rotates, points will be reached at which the blade is acted upon by gravity such that the root will tend to shift within its associated rotor groove. Such movement of the blade roots may cause an effect known as "fretting" in which particles are worn from surfaces which rub together which particles may oxidize and harden whereby they can abrade the blade root and associated rotor groove bearing surfaces. Loose fitting blades therefore have the potential to reduce the useful blade life in the root area due to fatigue.
For this reason, efforts have been made to prevent such relative motion between the blade root and its associated rotor groove. An example of one such effort may be found in U.S. Pat. No. 5,236,309 to Van Heusden et al., issued Aug. 17, 1993, assigned to the assignee of the present invention, and incorporated herein by reference. The '309 patent, as exemplified by FIG. 4, teaches utilizing belleville spring washers 8' compressed between the root bottom 12' and the groove base rotor 10' to keep the contacting surfaces in tight fitting arrangement. This has been found effective in controlling the problem, however, it has been noted that shims currently in use as an aide to the spring washers have the potential of slipping from their place of engagement between the spring washers and groove base whereby the rotor blade once again may become loose within the groove thereby subjecting the machine to damage caused by fretting. This is particularly true in that there currently is no discernable way on ensuring that the shim has been properly installed between the disc spring and the rotor groove base.
Therefore, an object of the present invention is to provide a shim design which will ensure that the shim will not become expelled during operation whereby the blade may become loose within the operating machine.
A further object of the present invention is to provide a discernable signal during assembly whereby confirmation of correct installation of the shim may be made.